The present invention relates to power switches, and in particular to power semi-conductor switches switching a load and, more particularly, to a circuit for preventing the reverse circulation of current in the body diode of power switches employing field effect transistors as the power switching devices.
In typical power switches as shown in FIG. 1, a power switch 10 comprising a MOSFET switches a load 20, for example, a motor. The power switch 10 includes the MOSFET portion 10A and a body diode 10B. The power switch 10 is driven by a driver 15. In a typical automotive application, the power switch switches the battery voltage VDD across the load 20, as shown in FIG. 1. It is possible that the load, for example a motor, may operate to produce a back EMF, for example, if the motor acts as a generator, which is higher than the battery voltage VDD. In such cases, a reverse current indicated by R will flow in the circuit through the body diode 10B of the FET power switch if the power switch is off because a body diode 15B which is contained in the driver output stage transistor 15A will conduct. The driver stage is typically tied to a substrate voltage S2 which can float about the voltage Vss. As a result, if the voltage S2 reaches a voltage that is 0.6V above VDD, the body diode 15B will conduct, causing power switch 10 to be off, and allowing a reverse circulation current to flow in body diode 10B of power switch 10. This causes greater heating than would occur through the power MOSFET in parallel with the body diode, and may damage the MOSFET.